1. Field of the Invention
The invention relates to computer systems with disk drives having motor driven head positioners wherein the drives are preadjusted to access data tracks at standard locations. Said drives are subject to the effects of wear, aging, mechanical drift and corrosion resulting in the need for periodic maintenance and realignment. In particular, floppy disk drives are especially sensitive to alignment problems because they are expected to maintain alignment within the limits required for interchangeability. Said limits are governed by accepted standards, e.g. ANSI X3.82-1980.
A machine readable alignment test disk offers the computer user or owner a means to verify correct adjustment of said drive without the need to remove it for testing, to disassemble the system, or to employ the services of a trained technician with special equipment.
2. Discussion of Prior Art
Several attempts have been made to measure the alignment of disk drives using prerecorded disks. U.S. Pat. No. 4,513,331 issued to Baker, et al. uses alternate offset and progressive offset sectors along a track to provide reference signals for testing head position. U.S. Pat. No. 4,458,275 issued to Monti, and U.S. Pat. No. 4,562,494 issued to Bond use progressive offset tracks to provide reference signals. Pat. No. 4,608,618 issued to Sturtevant-Stuart uses combinations of spiral tracks to provide reference signals.
By prerecording a suitable arrangement of special tracks on these diagnostic or alignment disks, it is taught that radial alignment, disk centering, mechanical hysteresis, and other parameters of interest can be adequately measured by persons with no special equipment or training.
All of the above methods rely on eventual failure of the playback system to correctly read prerecorded sectors in the presence of incrementally decreasing signal amplitude. The assumption is that signal amplitude threshhold detection provides a sufficiently stable reference for subsequent position calculations. However, since disk drives differ appreciably in their dynamic recovery threshholds, and because magnetic disks differ in absolute signal amplitude from one to the other, a two-point measurement is required. Typically, one measurement is taken with the reference sectors or tracks progressively offset toward the center of the disk until a failure occurs, and a second measurement is taken with the reference sectors or tracks progressively offset toward the outside of the disk until a similar failure occurs. By this means the nominal center for the head under test is computed to be half-way between the relative centers of the reference sectors or tracks at which failures were encountered. Briefly, the factors which affect signal amplitude are expected to be the same at both measurement points, hence the two points are alleged to represent equal off-track amounts.
Nevertheless, even after the two-point measurement is made, a significant source of error remains in all the above cited methods. This residual error exists because magnetic disks are not absolutely uniform in respect to those parameters which affect signal amplitude. Signal amplitude variations will occur due to differences in coating thickness across the surface of the disk, due to variations in average magnetic particle density in the coating mix, and due to variations in the average magnetic particle orientation on the disk. These amplitude variations, commonly called envelope modulation, will cause differences in the failure threshholds at the two points used for determining alignment. Furthermore, the component of signal amplitude variation due to non-uniformity of the disk cannot be distinguished from the component of amplitude variation due to track offset. Thus, there is an inherent, non-removable error associated with all these two point measurement methods. A problem with signal threshhold measurements is cited in U.S. Pat. No. 4,513,333 issued to Young et al., however the solution proposed there does not provide a measure of alignment, but rather provides pass/fail results based on a comparison of the head alignment with predetermined acceptable limits. Furthermore, any extensions of the method to allow making an approximate measure of alignment still require multiple point measurements.
A second problem with the above methods which affects parameter measurements that require readings to be taken on multiple tracks is that the signal amplitude from a disk varies in proportion to the track radius. This variation in amplitude conforms with known physical laws relating the output of a magnetic transducer (head) to the linear velocity of the media under the head. Said variations are not discussed in any of the prior art specifications and no compensation methods are cited. Absent any compensation, however, the signal loss caused by changes in track radius cannot be distinguished from signal loss caused by track offset.
A third problem which affects all the above methods is that the alignment measurement is quantized, i.e. the offsets are multiples of some basic incremental distance. Each incremental change in offset corresponds with a sector or other block of data. Hence, whatever arrangement of tracks and sectors are chosen, only a finite and relatively small number of measured values can be deduced.